The present invention relates generally to a personnel neutron dosimeter and more particularly, to a portable dosimeter capable of providing both an immediate indication of the neutron dose equivalent received by the person wearing the dosimeter and an alarm when that dose equivalent exceeds a selectable preset level.
Protection of workers in a radioactive environment requires an accurate and timely monitoring of the radiation dose equivalent received by each worker. Monitoring dose equivalents received for neutron exposures must take into account not only the radiation quantity but also the radiation quality. Unlike x, gamma, or beta radiations for which the hazards are substantially the same per unit of absorbed dose for the commonly encountered energies, neutron radiation can result in a hazard which increases with both increased unit absorbed dose and neutron energy. For a device to measure accurately the neutron dose equivalent received by a person exposed to unknown or varying spectra of neutron energies, the device not only should count the neutron events but also should compensate properly for the variability in hazard as a function of the neutron energy. To monitor the dose equivalent in a timely manner, the device should indicate the accumulated dose equivalent at any desired time and provide a warning when the accumulated dose equivalent reaches a chosen action level.
Several devices are commonly used to measure neutron dose equivalents received by personnel in radioactive environments. One such device is a badge containing neutron sensitive film as the measurement medium. Neutrons impinging on the film may strike a hydrogen atom in the film emulsion. This hydrogen atom is ionized into a proton, which then causes an ionization recoil track in the emulsion. Development of the film forms an image of the proton recoil track which may be visually detected. Evaluation of the accumulated dose equivalent received by the person wearing the badge can be made by counting the proton recoil tracks on the film, usually manually with the aid of a microscope.
Another commonly used device is a badge containing thermoluminescent dosimeters (TLD) as the measurement medium. These TLD are crystalline materials, usually containing the isotopes lithium-6 or boron-10, and have the property of luminescing when they are heated to a high temperature if they had previously been exposed to radiation. Both lithium-6 and boron-10 have a significant cross-section for the (n,.alpha.) reaction, whereby the alpha causes ionization in the crystal, imparting energy to the electrons. A portion of these electrons are trapped until the crystal is heated to a temperature sufficient to release them. The release of a trapped electron is accompanied by a flash of light (luminescence). The neutron dose equivalent is then evaluated from measurement of the light output when the TLD are evaluated by heating.
Other devices have been developed for use as personnel neutron dosimeters, including badges that contain fission track etch foils, proton recoil etch foils, or combinations of track etch foils and TLD, as the measurement medium.
These devices suffer, in various degrees, a disadvantage in that they do not measure accurately the dose equivalent received by a person exposed to unknown or varying spectra of neutron energies; i.e., the response of the device does not adequately indicate the neutron dose equivalent for all commonly encountered energies or spectra of neutrons. For example, the neutron film and proton-recoil track etch dosimeters are not sensitive to neutrons with energies less than approximately 200 keV. TLD are preferentially sensitive to thermal and low energy neutrons. Fission track etch dosimeters are sensitive only to neutrons with energies at which the fission cross-section is significant. Combinations of track etch dosimeters and TLD utilize the high energy response of the track etch dosimeter and the low energy response of the TLD to ameliorate the degree of this disadvantage.
All these devices suffer a major disadvantage in that they do not provide a direct readout of the accumulated dose equivalent in a timely manner; that is, the badge must be removed from the wearer and be developed before a reading is provided. Similarly, no alarm is provided to alert the wearer that his accumulated neutron dose equivalent has reached an action level. Accordingly, the wearer may be advised, after the fact, that he has been exposed to an excessive level of neutron radiation, but he is not warned at the time of exposure that he must take additional measures to protect himself.
One particular attempt to provide a portable, direct reading, neutron counter is described in an article entitled, "A Pocket-Sized Integrating Neutron Counter" by the present inventors and Valens P. Johnson, Rocky Flats publication RFP-794, Jan. 22, 1971. The detector described in this article used a lithium-6 foil mounted on a cadmium foil. The output of the detector was amplified and fed into a mono-stable multivibrator to provide an output pulse to drive a current amplifier. The output of the amplifier was then used to cause a small quantity of mercury to transfer across an electrolytefilled gap between mercury columns in a capillary tube of a microcoulometer, causing a displacement of the position of the gap. The net displacement of this gap was proportional to the number of neutron events detected, providing a visible indication of the number of pulses accumulated.
Tests of this counter showed that the sensitivity of the counter increased as the average neutron energy decreased, thereby providing a misleading increase in count rate when receiving less dangerous radiation. In addition, the counter showed a reading of approximately 1.4 mrem per day from background and gamma response. And most significantly, the device proved to be very sensitive to mechanical shocks which caused a displacement of the gap in the mercury column, thereby destroying the measurement of accumulated neutron dose.